It is often desirable to monitor the conditions inside a pneumatic tire while the tire is installed and in use on a vehicle. Examples of conditions to be monitored include pressure and temperature. Difficulties arise when trying to gain access inside the tire without removing the tire for inspection. Further, it is very difficult to apply a measurement device to the valve stem of a tire while it is rotating. Therefore, various systems have been developed to provide a wireless sensor on or inside the tire.
Large off-highway vehicles such as mine haul trucks and wheel loaders provide an additional incentive to provide a suitable wireless monitoring solution. Tire replacement cost for vehicles such as these are typically tens of thousands of dollars, and early detection of a tire failure is essential in preventing permanent damage. Further, these vehicles operate on dirt roads where tire-damaging rocks and other debris are common.
Autonomous vehicles provide a further incentive for providing a suitable wireless monitoring solution. An example of such a feature would be an autonomous haul truck used for mining applications. By nature, there is no operator present that could detect problems with a tire during operation. A tire monitoring system is therefore required to prevent operation of the vehicle after tire deflation or damage.
Mounting a suitable monitoring device onto or inside the tire has been a known challenge. A monitoring device mounted outside the tire or rim is subject to being struck by debris and damaged. A monitoring device mounted inside the tire or rim is subject to different environmental challenges. These include high pressure, high oxygen content, and exposure to liquids designed to increase tire performance. Further, methods of attaching a monitoring device inside the tire present challenges. One solution is to attach the monitoring device to the rim by connecting the monitoring device to a band that is wrapped around the circumference of the rim. However, some vehicle rims do not have a recessed section of the rim to create a cavity for the band connection solution. Thus, the monitoring device would be knocked off the rim by the tire bead when the tire is installed on the rim. Another solution is to attach the monitoring device to the inside of the tire. However, it is known that the tire flexes as it contacts the ground and supports the vehicle. The flexing of the tire therefore makes attachment difficult.
One solution is disclosed by U.S. Pat. No. 6,082,192 to Koch et al on Jul. 4, 2000. Koch et al discloses a spherical monitoring assembly that is inserted into the air chamber formed between the tire and rim. The curvature of the assembly is sufficient to enable the assembly to roll on the inside of the tire. However, the '192 patent has no provision to communicate temperature changes from outside the assembly to the sensor. Further, the transfer gel used in the '192 patent is not robust enough to withstand the shock, vibration, and high-g loads seen in the intended application. Still further, the use of a breathing tube limits communication of ambient conditions to the sensor and makes attachment to the sensor inconvenient.
U.S. Pat. No. 6,543,277 to Koch et al on Apr. 8, 2003 discloses a spherical monitoring assembly that has a specific gravity of less than one such that it will float on liquids contained within the tire. The '277 patent also discloses an external weight adapted to position the sensor in a predictable configuration, i.e. such that the sensor inlet is exposed to air instead of liquid. However, the external weight is problematic for at least two reasons. First, the external mass that enables a predictable configuration when floating is poorly suited to allowing the monitoring assembly to roll on the inside of the tire in the absence of liquid. The resulting poor balance will impart undue shock and vibration on the monitoring assembly, much as an unbalanced tire does when rolling down the road. Second, the external weight will cause the monitoring assembly to jump off of the inside of the tire whenever the external mass contacts the inside of the tire.